Field of the Invention
Embodiments of the present disclosure generally relate to the process of fabricating integrated circuits on substrates. More specifically, embodiments of the present disclosure describe processes and related apparatus for depositing low-k dielectric layers onto substrates for manufacturing integrated circuit devices.
Description of the Related Art
For decades, the semiconductor industry has worked toward producing integrated circuits (IC's) that are smaller, faster, and containing more devices than the IC's formed previously. Reduction of dielectric constants in low-k insulating materials is part of this effort. Low-k materials are typically considered those which have a dielectric constant, or k, of less than 4. The constant k is defined as the ratio of a material's permittivity compared to that of vacuum. Different approaches for reducing k have included using organic polymer materials, adding fluorine or carbon or other materials to silicon dioxide, and incorporating pores into dielectric layers.
One example of a material which combines two k-reducing strategies, a silicon oxide containing carbon and nanometer-scale pores, is Applied Materials' Black Diamond 3™ film. A starting layer can be deposited in a Black Diamond 3™ PECVD chamber, after which pores can be created by a curing treatment which removes porogens formed in a deposited layer. Pores can reduce the film's k value by as much as 30%. A typical curing treatment consists of exposing the deposited film to ultraviolet (UV) radiation and annealing. A curing process removes the porogen material to form empty pores while simultaneously cross-linking the bulk layer, but some of the remaining porogen material may be trapped in the bulk layer structure. During the cross-linking process, silicon-oxygen-silicon chains are formed, which creates a bulk-layer bonding structure and thus a mechanically strong layer. The cured Black Diamond 3™ layer's pores have a characteristic size distribution and are physically isolated so that the material remains sufficiently strong.
Porogens formed in the deposited layer occupy a volume in the formed layer which would otherwise be occupied by the bulk layer material. Porogens are typically selected from materials which can be decomposed into byproducts which can be removed by a conventional curing process in order to form a void or pore in the bulk layer material. Preferably the decomposed porogen material is diffused completely out of the layer, volatized, and removed from the processing region formed above the bulk layer.
Curing simultaneously cross-links or densifies the bulk layer and decomposes the material within pores, or porogens. As porogens are removed to form pores, the k value of the formed layer is decreased, and as the silicon bonds are cross-linked, the material becomes stronger. However, cross-linking also reduces the rate of diffusion of porogenic materials from the deposited layer. Therefore, as the cross-linking process is completed, any remaining porogens are trapped within the layer. Thus, longer curing times become ineffective in removing the porogens to form additional pores.
When the bulk layer material is completely cross-linked near the end of a curing process, remaining porogens are trapped within the bulk layer. It is believed that the remaining porogens can increase a film's dielectric constant, compared to a film not containing porogens at the end of a curing process. It is also believed that the remaining porogens may reduce a film's mechanical strength.
A porous dielectric layer's k value can be reduced simply by incorporating a larger volume percent of pores within a layer. However, higher porosity results in reduced mechanical strength. Thus, there is a tradeoff between a layer's low-k properties versus the layer's mechanical strength. For example, some low-k films require sufficient mechanical strength for surviving processes, such as chemical-mechanical polishing (CMP) processes that are typically performed on substrates that contain IC devices. CMP machines can apply large forces to the surface layers of a substrate, which can damage a mechanically weak layer.
Therefore there is a need for methods of forming dielectric layers which can have a reduced k value and have a desirable mechanical strength.